Natural gas hydrates are cage-type complex compounds, and have characteristics including abundant reserve, wide distribution, high energy density, and clean combustion, etc. Therefore, natural gas hydrates are regarded as a new alternative energy resource that has high potential in the twenty-first century. Natural gas hydrates are formed by small molecular gasses bonded with water molecules under high pressure and low temperature, and the small molecular gasses mainly include hydrocarbon gasses, such as methane and ethane, etc. It is found in the researches over the years: the natural gas hydrates in the nature are mainly distributed in polar tundra in high-latitude regions and in deep sea beds, continental slopes, continental rises, and ocean trenches around the world, wherein, the natural gas reserve occurred in marine hydrates is estimated to be 20×1015 m3. Therefore, it is of far reaching importance to exploit and utilize marine hydrate resources rationally to mitigate the energy crisis and enrich the energy structure.
Up to now, relatively matured hydrate recovery methods mainly include thermal stimulation, depressurization, chemical inhibitor injection, and carbon dioxide (CO2) replacement. Wherein, the depressurization is usually applied to change the phase equilibrium of the hydrates by decreasing the pressure in the reservoir system, and thereby induce the dissociation of natural gas hydrate with the advantages including low cost, easy operation, and low energy loss. However, it is too slow for commercial exploitation and hard to be controlled. Chemical inhibitor injection is to inject chemical agents, such as alcohols and brines, etc., to drive the natural gas hydrate to decompose with the advantages including easy and simple operation and significant effect. However, it cost too much and always pollutes the environment. Carbon dioxide (CO2) replacement is to use CO2 to replace methane gas in the natural gas hydrate by utilizing a fact that CO2 can form hydrates more easily than CH4, and it has advantages that double benefits are gained (i.e., the greenhouse gas CO2 is sequestrated while the natural gas is recovered) and the stability of the reservoir is maintained at the same time, but has drawbacks including low cost-performance and low exploitation rate. Thermal stimulation usually is to increase the temperature of the reservoir system by injecting hot fluid or by electromagnetic heating, and thereby drive the natural gas hydrates to decompose, and it has advantages including convenient operation, high exploitation rate and high controllability, but has drawbacks including high cost, high energy loss, and low heat utilization efficiency.
Therefore, it is desirable to develop an apparatus and a method for marine hydrate reserve recovery, which have advantages including high exploitation rate, low production cost, low energy loss, and high heat utilization efficiency.